BACKGROUND: Operational Tolerance (opTOL) is deemed to be extremely rare in kidney transplantation. We launched a Europe-wide survey to assess the frequency of opTOL.

METHODS: 18 coordinators in 27 countries were recruited through a European consortium. They sent a questionnaire to 246 transplant centers to identify tolerant (defined as having a S creat <1.7 mg/dL w/o glomerular proteinuria despite at least one year w/o any immunosuppressive drug) or almost tolerant patients (same criteria as above but with minimal immunosuppression: prednisone <10 mg/day). They also recorded the number of kidney recipients, tolerant or almost tolerant patients ever followed at each center.

RESULTS: 140 questionnaires covering 202,643 transplants were returned, which identified 63 tolerant and 77 almost tolerant patients. Hence, tolerance and almost tolerance were declared in 1/3,333 and 1/2,500 kidney transplantations, respectively. 22 of the 63 tolerant patients were previously reported. We had insufficient data in 6. Therefore, we describe 35 new tolerant patients, mostly males (n= 26/35) who withdrew immunosuppressive medications for noncompliance (n= 28/32). They displayed a median HLA mismatch number of 2 (IQR 0-3). The mean follow-up was 203±95 months. The mean duration of tolerance, according to our strict criteria, was 75±55 months. The mean duration of a functioning transplant not requiring dialysis w/o any immunosuppressive drug, irrespective of creatinine, was 88±63 months. At the end of the observation period, 31/35 patients were alive and 24/31 still tolerant. The remaining 7/31 had rising creatinine of which 4 resumed dialysis.

CONCLUSION: We confirmed that incidence of OpTOL in kidney transplantation is low, declared as 1/3,333, but also that tolerance is only a metastable status with patients losing their graft or harboring graft dysfunction over time.

*The two first and senior authors contributed equally.

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